A method for alternately building-up an insulating layer and a conductor layer in a core substrate is adopted at present to realize an increase in density of the multilayer printed wiring board. Here, there are two kinds of methods constructed by full additive and semi-additive methods as the built-up method. A manufacturing process of a conductor circuit onto an interlayer resin insulating layer of the multilayer printed wiring board using this semi-additive method will be explained next with reference to FIG. 32.
First, an insulating layer 250 having an opening 250a as a via hole is formed on each of both faces of a core substrate 230. An electroless plating copper film 252 is uniformly formed on a surface of the interlayer resin insulating layer 250 (FIG. 32(A)). An unillustrated resist film for forming a resist is adhered onto the electroless plating copper film 252. Thereafter, the resist film is exposed and developed and a resist 254 for plating is formed (FIG. 32(B)). Thereafter, an electrolytic plating copper film 256 is deposited in a nonforming portion of the resist 254 by dipping the core substrate 230 into an electrolytic plating liquid and flowing an electric current through the electroless plating copper film 252 (FIG. 32(C)). Wiring patterns 258a, 258b and a via hole 260 are then formed by separating the resist 254 and separating the electroless plating copper film 252 below the resist 254 by etching. Similar processes are repeated and an interlayer resin insulating layer 350, a wiring pattern 358 and a via hole 360 are further formed (FIG. 32(E)).
FIG. 33(B) shows a B-B section of FIG. 32(E). At present a design of pulling-out wiring branched from one main wiring is adopted to prevent disconnection in the multilayer printed wiring board. Therefore, a crossing portion X formed in a T-character shape is formed as shown in FIGS. 33(A) and 33(B).
However, there is a case in which the wiring pattern is disconnected in the above crossing portion X. Namely, the wiring pattern 258 is formed in the nonforming portion of the resist 254 as mentioned above with reference to FIG. 32(C). However, as shown by the crossing portion X in FIG. 33(A), no plating liquid can be sufficiently moved around a corner portion C in which wall faces 258β, 258β of the wiring pattern 258 in the crossing portion cross at an angle (here a right angle) equal to or smaller than 90°. Accordingly, the wiring pattern is made thin so that disconnection is caused in a particular case.
Further, as shown in FIG. 33(B), the wiring pattern 258b formed by a metal such as copper, etc. is suddenly curved in the crossing portion X. Therefore, a case in which stress is concentrated to the corner portion C of the crossing portion in repetition of thermal contraction and a crack CL is thereby caused in the wiring pattern and the wiring pattern is thus disconnected.
Furthermore, when the wiring pattern 258b is coated with the interlayer resin insulating layer 350 as mentioned above with reference to FIG. 32(E), there is a case whereby an air bubble B is left between the wiring pattern 258b and the interlayer resin insulating layer 350 in the corner portion C of the crossing portion as shown in FIG. 33(B). At this point, when the air bubble B is left in a lower layer of the interlayer resin insulating layer 350, the air bubble B is expanded in the thermal contraction of the printed wiring board and causes a breakdown of the printed wiring board.
In a further background art of the present invention, a multilayer build-up wiring board is formed by alternately laminating an interlayer resin insulating layer and a wiring layer on a core substrate. The multilayer build-up wiring board is mainly manufactured by an additive method at present. The above wiring layer is formed in an opening portion of a resist formed on the interlayer resin insulating layer by electrolysis or by electroless plating. Upper and lower wiring layers are electrically connected to each other by a via hole extending through the interlayer resin insulating layer. Each of these wiring layers is constructed by a via hole land used as a receiving pan of the via hole, a wiring pattern, a solid portion having a high potential applied by a power source, etc. and having a function similar to the function of a capacitor electrode, etc. Here, minimum values of a size of the via hole land, a width of the wiring pattern and an insulating distance between the via hole land and the wiring pattern are determined by resolution of the resist, an attaching degree of plating, etc. The via hole land and the wiring pattern are manufactured by setting the size, the width and the insulating distance to be greater than these minimum values respectively.
The multilayer build-up wiring board for a package functions as a connector for electrically connecting an electronic part such as an IC chip, etc. mounted to an upper face of the multilayer build-up wiring board to a printed wiring board such as a mother board, etc. located on a lower face of the multilayer build-up wiring board. Here, it is required that a line width of the wiring pattern, an insulating distance and a land diameter are reduced to cope with an increase in density of a connecting portion of the electronic part and the printed wiring board. However, when these values are set smaller than the above minimum values respectively, no desirable wiring can be formed by dispersion of a slight process condition in order that the probability of generation of disconnection of the wiring, a short-circuit of wirings, etc. is increased and yield is reduced.
In contrast to this, it is also possible to cope with the above increase in density by increasing the number of build-up layers of the multilayer build-up wiring board without reducing the line width of the wiring pattern and the insulating distance. However, if the number of build-up layers is increased, a manufacturing process becomes exponentially complicated and reliability and yield are reduced.
Here, in the further background art of the present invention, thick and thin portions of the wiring pattern are formed in the multilayer build-up wiring board of the prior art so that resistance is not uniform and has a inferior influence on propagation of an electric signal. Further, no thickness of an interlayer resin insulating layer (30 μm) formed on an upper layer of the wiring pattern (having 15 μm average in thickness) is uniformed so that no electric characteristics of the wiring board can be constantly set. Therefore, it is difficult to improve performance of the multilayer build-up wiring board.
When the inventors of this application investigated this cause, it was found that the thickness of the interlayer resin insulating layer was dispersed by an arranging density of the wiring pattern. For example, there is a case in which the thickness of the interlayer resin insulating layer is thin in a high wiring density portion and is thick in a low wiring density portion (having no signal line therearound). In contrast to this, there is also a case in which the thickness of the interlayer resin insulating layer is thick in a high wiring density portion and is thin in a low wiring density portion.
It is considered from these facts initially that the thickness of the interlayer resin insulating layer is dispersed by plating thickness. In particular, it is considered that the thickness of a signal line is increased in the low wiring density portion since an electric field is concentrated to this low wiring density portion in electrolytic plating. In contrast to this, it is considered that the thickness of the signal line is reduced in the high wiring density portion since the electric field is dispersed.
Furthermore, for a second reason it is considered that the thickness of the wiring pattern is dispersed by a moving-round of an etching liquid. The multilayer build-up wiring board at present is mainly formed by the semi-additive method to obtain higher performance. In the semi-additive method, after an electroless plating film is uniformly formed in the interlayer resin insulating layer, a resist pattern is formed and a conductor layer is formed by forming an electrolytic plating film in a nonforming portion of the resist by flowing an electric current through the electroless plating film. Here, after the electrolytic plating film is formed and the resist is separated, the electroless plating film below the resist is removed therefrom by light etching. However, in this light etching, no etching liquid can be sufficiently moved around the high wiring density portion so that the thickness of the wiring pattern can be increased. In contrast to this, the etching liquid is moved around the low wiring density portion excessively so that the thickness of the wiring pattern is reduced and a line width is also narrowed in a particular case.
In a further background art of the present invention, a conductor layer within a multilayer core substrate and a build-up wiring layer in a package substrate in the prior art are connected to each other by arranging an inner layer pad wired to a surface of the multilayer core substrate from a through hole and connecting a via hole to this inner layer pad. Namely, the inner layer pad for connecting the via hole to an upper layer is added to a land of the through hole, or the inner layer pad for connecting the via hole is connected to the land of the through hole through wiring.
However, in a land shape of the prior art, a through hole distance is widened to hold mutual insulation of the inner layer pad and the number of through holes formed in the multilayer core substrate is limited by this land shape.
In contrast to this, the number of bumps formed on a rear face of the package substrate is set to be greater than the number of bumps formed on a front side of the package substrate. This is because wirings from plural bumps on the rear face are connected to the bumps on the front side while these wirings on the rear face are integrated with each other. For example, power lines requiring low resistance to a signal line are set to 20 lines in the bumps on the rear face (on a mother board side), but are integrated as one line on the front face (an IC chip side).
Here, it is desirable to set the number of upper build-up wiring layers and the number of lower build-up wiring layers to be equal to each other, i.e., minimize the layer numbers that wirings can be integrated with each other at the same pace in build-up wiring layers formed on the front side of the core substrate and build-up wiring layers formed on the rear side of the core substrate. However, as mentioned above, the number of through holes able to be formed in the multilayer core substrate is limited. Therefore, in the package substrate of the prior art, wirings are integrated with each other to a certain extent in the build-up wiring layers on the rear side and are then connected to the build-up wiring layers on the front side through the through holes of the multilayer core substrate. Namely, since the wiring density is reduced in the build-up wiring layers on the front side, the same layer number as the number of build-up wiring layers on the rear side is not originally required. However, when the number of build-up wiring layers on the front and rear sides are set to be different to each other, a warp is caused by an asymmetric property. Therefore, the number of build-up wiring layers on the front and rear sides are set to be equal to each other. Namely, since the number of through holes formed in the multilayer core substrate is limited, the number of build-up wiring layers on the rear side must be increased and build-up wiring layers on the front side must be further formed to such a degree that the number of build-up wiring layers on the front side is equal to the increased number of build-up wiring layers on the rear side.
Namely, in the printed wiring board (package substrate) of the prior art, since the number of build-up layers is increased, problems exist whereby the connection reliability of upper and lower layers is reduced and the cost of the package substrate is increased and, thickness and weight of the package substrate is excessively increased.
To solve the above-mentioned problems, an object of the present invention is to provide a printed wiring board and a manufacturing method of the printed wiring board in which there is no disconnection caused in a wiring pattern.
An object of the present invention is to provide a printed wiring board of high density which is able to be manufactured with a high yield.
An object of the present invention is to provide a printed wiring board having superior uniform properties of thickness of a wiring pattern and an interlayer resin insulating layer.
An object of the present invention is to provide a printed wiring board which is able to reduce the number of build-up layers by increasing the density of through holes formed in a core substrate and to provide a manufacturing method of the printed wiring board.